1. Field of the Invention
The present invention relates to a radiation detector used in, for example, a medical X-rays imaging apparatus.
2. Description of the Related Art
Recently, a radiation detector such as an FPD (Flat Panel Detector) capable of directly converting radiation to digital data, has come into practical use. The radiation detector has a merit in that it can confirm an image at once in comparison with a conventional imaging plate. The radiation detector having the merit has rapidly become widespread.
Various types of radiation detectors have been proposed. Among such radiation detectors, there is, for example, a direct-conversion-type radiation detector that directly converts radiation to charges by a semiconductor layer and that stores the charges. Further, there is an indirect-conversion-type radiation detector that converts radiation to light once by a scintillator such as CsI: Tl, and GOS (Gd 2O2S: Tb), then converts the converted light to charges by a semiconductor layer, and that stores the charges.
A radiation detector has a bias electrode which is disposed on one surface of a semiconductor layer, formed on an insulating substrate, and applies a bias voltage. Together therewith, the radiation detector has plural collecting electrodes which are disposed on the other surface of the semiconductor layer, that collect charges. The radiation detector causes the semiconductor layer to generate an electric field by applying a bias voltage to the bias electrode. Further, the radiation detector collects the charges generated in the semiconductor layer by the collecting electrodes, and reads out the charges as information representing a radiation image.
An electrostatic capacitance (so-called, parasitic capacitance) is generated in an insulating substrate by an electric field generated by applying the bias voltage to the bias electrode, and thereby charges are generated. The charges generated in the insulating substrate changes by a peripheral environment such as temperature and humidity, the bias voltage, and an application time in which the voltage are applied. The change of the electrostatic capacitance generated between the insulating substrate and the collecting electrodes affects reading of the charges. In particular, many collecting electrodes are disposed in periphery the central portion of the semiconductor layer of the insulating substrate, opposing with the bias electrode. Therefore, since charge amount generated is small, the image is less affected by charges. However, in an edge portion of the semiconductor layer of the insulating substrate, only a small number of collecting electrodes is disposed opposing with the bias electrode. Further, a wrap-around of the electric field also occurs in the edge portion of the semiconductor layer. Therefore, the charge amount generated is large, and the image may be affected by charges.
As a technique for suppressing an influence to an image due to the change of the electrostatic capacitance, Japanese Patent No. 3818271 discloses a technique for preventing a change of an electrostatic capacitance between a bias electrode and an upper cabinet that protects a radiation detector. In this technique, a sealed member composed of a nonconductive material is disposed on the upper portion of the bias electrode (opposite side of a semiconductor layer).
Further, Japanese Patent Application Laid-Open (JP-A) No. 2001-53327 discloses a technique for preventing noise generated in a signal line by a capacitor coupling. In the technique, guard electrodes are disposed, in the peripheral portion of the semiconductor layer, between signal lines to which signals of charges collected by collecting electrodes flow, and a semiconductor layer. The technique of JP-A No. 2001-53327 prevents the capacitor coupling between the semiconductor layer and the signal lines. Thus, in the technique of JP-A No. 2001-53327, the guard electrodes are formed between the signal lines disposed on the semiconductor layer side of a substrate and the semiconductor layer.
Further, Japanese Patent No. 3311273 discloses a technique for preventing occasion of a dielectric breakdown caused by applying a high voltage to collecting electrodes from a peripheral portion of a semiconductor layer, by disposing dummy electrodes at peripheral portions. In this technique, the dummy electrodes are formed around the collecting electrodes (on the semiconductor layer side of a substrate) to prevent the high voltage from being applied to the collecting electrodes from the peripheral portion.
However, the technique of Japanese Patent No. 3818271 can not suppress a deterioration of image quality due to the change of the electrostatic capacitance between the collecting electrodes and the insulating substrate.
Further, there is considered a configuration for preventing the change of the electrostatic capacitance by disposing guard electrodes between signal lines and a semiconductor layer in the peripheral portion of a semiconductor layer, and by disposing dummy electrodes in the peripheral portion of the semiconductor layer, as disclosed in JP-A No. 2001-53327 and Japanese Patent No. 3311273. However, when the electrodes are disposed on the semiconductor layer side of a substrate, the electrodes must be disposed externally away with a predetermined distance from the collecting electrodes of the peripheral portion, to prevent discharges between the electrodes and the collecting electrodes. As a result, the size of a radiation detector increases.